Method for making a well disposed over a sensor

ABSTRACT

A method for forming a well providing access to a sensor pad includes patterning a first photoresist layer over a dielectric structure disposed over the sensor pad; etching a first access into the dielectric structure and over the sensor pad, the first access having a first characteristic diameter; patterning a second photoresist layer over the dielectric structure; and etching a second access over the dielectric structure and over the sensor pad. The second access has a second characteristic diameter. The first and second accesses overlapping. A diameter ratio of the first characteristic diameter to the second characteristic diameter is not greater than 0.7. The first access exposes the sensor pad. The second access has a bottom depth less than a bottom depth of the first access.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This is a continuation application under 35 U.S.C. § 120 of pending U.S.application Ser. No. 15/945,707 filed Apr. 4, 2018, which applicationsclaims benefit under 35 U.S.C. § 119(e) of U.S. Provisional ApplicationNo. 62/481,610 filed Apr. 4, 2017. The entire contents of theaforementioned applications incorporated by reference herein.

FIELD OF THE DISCLOSURE

This disclosure, in general, relates to methods for making a well over asemiconductor device.

BACKGROUND

A variety of types of chemical devices have been used in the detectionof chemical processes. One type is a chemically-sensitive field effecttransistor (chemFET). A chemFET includes a source and a drain separatedby a channel region, and a chemically sensitive area coupled to thechannel region. The operation of the chemFET is based on the modulationof channel conductance, caused by changes in charge at the sensitivearea due to a chemical reaction occurring nearby. The modulation of thechannel conductance changes the threshold voltage of the chemFET, whichcan be measured to detect or determine characteristics of the chemicalreaction. The threshold voltage can, for example, be measured byapplying appropriate bias voltages to the source and drain, andmeasuring a resulting current flowing through the chemFET. In anotherexample, the threshold voltage can be measured by driving a knowncurrent through the chemFET, and measuring a resulting voltage at thesource or drain.

An ion-sensitive field effect transistor (ISFET) is a type of chemFETthat includes an ion-sensitive layer at the sensitive area. The presenceof ions in an analyte solution alters the surface potential at theinterface between the ion-sensitive layer and the analyte solution, forexample, from the protonation or deprotonation of surface charge groupscaused by the ions present in the analyte solution. The change insurface potential at the sensitive area of the ISFET affects thethreshold voltage of the device, which can be measured to indicate thepresence or concentration of ions within the solution. Arrays of ISFETscan be used for monitoring chemical reactions, such as DNA sequencingreactions, based on the detection of ions present, generated, or usedduring the reactions. More generally, large arrays of chemFETs or othertypes of chemical devices can be employed to detect and measure staticor dynamic amounts or concentrations of a variety of analytes (e.g.hydrogen ions, other ions, compounds, etc.) in a variety of processes.The processes can, for example, be biological or chemical reactions,cell or tissue cultures or monitoring neural activity, nucleic acidsequencing, etc.

An issue that arises in the operation of large scale chemical devicearrays is the susceptibility of the sensor output signals to noise.Specifically, the noise affects the accuracy of the downstream signalprocessing used to determine the characteristics of the chemical orbiological process being detected by the sensors. It is thereforedesirable to provide devices including low noise chemical devices, andmethods for manufacturing such devices.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure may be better understood, and its numerousfeatures and advantages made apparent to those skilled in the art byreferencing the accompanying drawings.

FIG. 1 includes an illustration of an exemplary system including asensor array.

FIG. 2 includes an illustration of an exemplary sensor and associatedwell.

FIG. 3 includes an illustration of exemplary methods for preparing asequencing device.

FIG. 4 is a block diagram describing an example system.

FIG. 5, FIG. 6, FIG. 7, FIG. 8, FIG. 9, FIG. 10, and FIG. 11 includeillustrations of example work pieces during a process for making amicrowell.

FIG. 12, FIG. 13, FIG. 14, FIG. 15, FIG. 16, FIG. 17, and FIG. 18include illustrations of example steps in a process for forming amicrowell.

The use of the same reference symbols in different drawings indicatessimilar or identical items.

DETAILED DESCRIPTION

In an example embodiment, a device includes a dielectric structuredisposed over an array of sensors. The dielectric structure definesmicrowells, which include a first access exposing a sensor pad of asensor and a second access overlapping the first access and having acharacteristic diameter larger than the characteristic diameter of thefirst access. Together, the first access and second access form themicrowell. In use, the microwell can provide a fluid path from a bulksolution through to the sensor pad. In particular, reactants or reactionbyproducts associated with reactions taking place within the microwellcan be measured using the sensor pad surface exposed by the firstaccess. Optionally, a conformal metal coating, such as a titaniumcoating, can be disposed within the first access or the second access ofthe microwell, extending the sensor surface to over the surface of thewell. Optionally, the dielectric structure has more than one layer. Forexample, the dielectric structure can have a silicon oxide layer, suchas a high density plasma chemical vapor deposited silicon dioxide layer.Further, the dielectric structure can have a nitride layer, such as asilicon nitride layer. In a further example, the dielectric structurecan have a silicon oxide layer, such as a tetraethyl orthosilicate(1LOS) deposited silicon dioxide layer.

In an example, a sensor pad of the sensor is disposed within adielectric structure. The dielectric structure can have a single layer,such as a high density plasma chemical vapor deposited silicon dioxide.In another example, the dielectric structure can have a multilayerstructure, such as a silicon oxide layer formed around the sensor pad, asilicon nitride layer disposed over the silicon oxide layer, and asilicon oxide layer, such as a layer formed using TEOS, disposed overthe silicon nitride layer. A photoresist can be patterned to define anopening, and the dielectric structure can be etched to form a firstaccess having a characteristic diameter associated with the opening. Asused herein, the characteristic diameter is the square root of fourtimes a cross-sectional area divided by pi (i.e., sqrt(4 A/pi)). In anexample, the first access can be etched partially through the dielectricstructure. In another example, the first access can be etched throughthe dielectric structure to expose a surface of the sensor pad. Thefirst photoresist layer can be removed and cleaned from the work piece,and a second photoresist layer can be formed and pattern to define asecond opening, larger in characteristic diameter than the firstopening. The dielectric structure can be etched to form a second accessoverlapping the first access and formed partially through the dielectricstructure. The second access does not extend to expose sensor pad.Optionally, prior to forming the second photoresist layer, a bottomanti-reflective coating is formed over the dielectric structure and atleast partially within the first access. The second photoresist layerand optionally the bottom anti-reflective coating (BARC) can be cleanedfrom the work piece. In a further example, a conformal metal layer canbe formed within the first access and the second access, for example,through sputtering or deposition. A conformal metal coating can beremoved from interstitial areas surrounding the second access. The firstaccess and the second access can define a microwell.

The device including the dielectric layer defining the microwell formedfrom the first access and second access and exposing a sensor pad findsparticular use in detecting chemical reactions and byproducts, such asdetecting the release of hydrogen ions in response to nucleotideincorporation, useful in genetic sequencing, among other applications.In a particular embodiment, a sequencing system includes a flow cell inwhich a sensory array is disposed, includes communication circuitry inelectronic communication with the sensory array, and includes containersand fluid controls in fluidic communication with the flow cell. In anexample, FIG. 1 illustrates an expanded and cross-sectional view of aflow cell 100 and illustrates a portion of a flow chamber 106. A reagentflow 108 flows across a surface of a microwell array 102, in which thereagent flow 108 flows over the open ends of microwells of the microwellarray 102. The microwell array 102 and a sensor array 105 together mayform an integrated unit forming a lower wall (or floor) of flow cell100. A reference electrode 104 may be fluidly coupled to flow chamber106. Further, a flow cell cover 130 encapsulates flow chamber 106 tocontain reagent flow 108 within a confined region.

FIG. 2 illustrates an expanded view of a microwell 201 and a sensor 214,as illustrated at 110 of FIG. 1. The volume, shape, aspect ratio (suchas base width-to-well depth ratio), and other dimensionalcharacteristics of the microwells may be selected based on the nature ofthe reaction taking place, as well as the reagents, byproducts, orlabeling techniques (if any) that are employed. The sensor 214 can be achemical field-effect transistor (chemFET), more specifically anion-sensitive FET (ISFET), with a floating gate 218 having a sensorplate 220 optionally separated from the microwell interior by apassivation layer 216. The sensor 214 can be responsive to (and generatean output signal related to) the amount of a charge 224 present onpassivation layer 216 opposite the sensor plate 220. Changes in thecharge 224 can cause changes in a current between a source 221 and adrain 222 of the chemFET. In turn, the chemFET can be used directly toprovide a current-based output signal or indirectly with additionalcircuitry to provide a voltage-based output signal. Reactants, washsolutions, and other reagents may move in and out of the microwells by adiffusion mechanism 240.

In an embodiment, reactions carried out in the microwell 201 can beanalytical reactions to identify or determine characteristics orproperties of an analyte of interest. Such reactions can generatedirectly or indirectly byproducts that affect the amount of chargeadjacent to the sensor plate 220. If such byproducts are produced insmall amounts or rapidly decay or react with other constituents, thenmultiple copies of the same analyte may be analyzed in the microwell 201at the same time in order to increase the output signal generated. In anembodiment, multiple copies of an analyte may be attached to a solidphase support 212, either before or after deposition into the microwell201. The solid phase support 212 may be microparticles, nanoparticles,beads, solid or porous comprising gels, or the like. For simplicity andease of explanation, solid phase support 212 is also referred herein asa particle. For a nucleic acid analyte, multiple, connected copies maybe made by rolling circle amplification (RCA), exponential RCA, or liketechniques, to produce an amplicon without the need of a solid support.

In particular, the solid phase support can include copies ofpolynucleotides. In a particular example illustrated in FIG. 3,polymeric particles can be used as a support for polynucleotides duringsequencing techniques. For example, such hydrophilic particles canimmobilize a polynucleotide for sequencing using fluorescent sequencingtechniques. In another example, the hydrophilic particles can immobilizea plurality of copies of a polynucleotide for sequencing usingion-sensing techniques. Alternatively, the above described treatmentscan improve polymer matrix bonding to a surface of a sensor array. Thepolymer matrices can capture analytes, such as polynucleotides forsequencing.

In general, the polymeric particle can be treated to include abiomolecule, including nucleosides, nucleotides, nucleic acids(oligonucleotides and polynucleotides), polypeptides, saccharides,polysaccharides, lipids, or derivatives or analogs thereof. For example,a polymeric particle can bind or attach to a biomolecule. A terminal endor any internal portion of a biomolecule can bind or attach to apolymeric particle. A polymeric particle can bind or attach to abiomolecule using linking chemistries. A linking chemistry includescovalent or non-covalent bonds, including an ionic bond, hydrogen bond,affinity bond, dipole-dipole bond, van der Waals bond, and hydrophobicbond. A linking chemistry includes affinity between binding partners,for example between: an avidin moiety and a biotin moiety; an antigenicepitope and an antibody or immunologically reactive fragment thereof; anantibody and a hapten; a digoxigen moiety and an anti-digoxigenantibody; a fluorescein moiety and an anti-fluorescein antibody; anoperator and a repressor; a nuclease and a nucleotide; a lectin and apolysaccharide; a steroid and a steroid-binding protein; an activecompound and an active compound receptor; a hormone and a hormonereceptor; an enzyme and a substrate; an immunoglobulin and protein A; oran oligonucleotide or polynucleotide and its corresponding complement.

As illustrated in FIG. 3, a plurality of polymeric particles 304 can beplaced in a solution along with a plurality of polynucleotides 302. Theplurality of particles 304 can be activated or otherwise prepared tobind with the polynucleotides 302. For example, the particles 304 caninclude an oligonucleotide complementary to a portion of apolynucleotide of the plurality of polynucleotides 302. In anotherexample, the polymeric particles 304 can be modified with targetpolynucleotides 304 using techniques such as biotin-streptavidinbinding.

In a particular embodiment, the hydrophilic particles andpolynucleotides are subjected to polymerase chain reaction (PCR)amplification or recombinase polymerase amplification (RPA). Forexample, dispersed phase droplets 306 or 308 are formed as part of anemulsion and can include a hydrophilic particle or a polynucleotide. Inan example, the polynucleotides 302 and the hydrophilic particles 304are provided in low concentrations and ratios relative to each othersuch that a single polynucleotide 302 is likely to reside within thesame dispersed phase droplets as a single hydrophilic particle 304.Other droplets, such as a droplet 308, can include a single hydrophilicparticle and no polynucleotide. Each droplet 306 or 308 can includeenzymes, nucleotides, salts or other components sufficient to facilitateduplication of the polynucleotide.

In a particular embodiment, an enzyme such as a polymerase is present,bound to, or is in close proximity to the hydrophilic particle orhydrogel particle of the dispersed phase droplet. In an example, apolymerase is present in the dispersed phase droplet to facilitateduplication of the polynucleotide. A variety of nucleic acid polymerasemay be used in the methods described herein. In an exemplary embodiment,the polymerase can include an enzyme, fragment or subunit thereof, whichcan catalyze duplication of the polynucleotide. In another embodiment,the polymerase can be a naturally-occurring polymerase, recombinantpolymerase, mutant polymerase, variant polymerase, fusion or otherwiseengineered polymerase, chemically modified polymerase, syntheticmolecules, or analog, derivative or fragment thereof.

Following PCR or RPA, particles are formed, such as particle 310, whichcan include the hydrophilic particle 312 and a plurality of copies 314of the polynucleotide. While the polynucleotides 314 are illustrated asbeing on a surface of the particle 310, the polynucleotides can extendwithin the particle 310. Hydrogel and hydrophilic particles having a lowconcentration of polymer relative to water can include polynucleotidesegments on the interior of and throughout the particle 310 orpolynucleotides can reside in pores and other openings. In particular,the particle 310 can permit diffusion of enzymes, nucleotides, primersand reaction products used to monitor the reaction. A high number ofpolynucleotides per particle produces a better signal.

In embodiments, polymeric particles from an emulsion-breaking procedurecan be collected and washed in preparation for sequencing. Collectioncan be conducted by contacting biotin moieties (e.g., linked toamplified polynucleotide templates which are attached to the polymericparticles) with avidin moieties, and separation away from polymericparticles lacking biotinylated templates. Collected polymeric particlesthat carry double-stranded template polynucleotides can be denatured toyield single-stranded template polynucleotides for sequencing.Denaturation steps can include treatment with base (e.g., NaOH),formamide, or pyrrolidone.

In an exemplary embodiment, the particle 310 can be utilized in asequencing device. For example, a sequencing device 316 can include anarray of wells 318. The sequencing device 316 can be treated with a washsolution including sulfonic acid, as described above. A particle 310 canbe placed within a well 318.

In an example, a primer can be added to the wells 318 or the particle310 can be pre-exposed to the primer prior to placement in the well 318.In particular, the particle 310 can include bound primer. The primer andpolynucleotide form a nucleic acid duplex including the polynucleotide(e.g., a template nucleic acid) hybridized to the primer. The nucleicacid duplex is an at least partially double-stranded polynucleotide.Enzymes and nucleotides can be provided to the well 318 to facilitatedetectible reactions, such as nucleotide incorporation.

Sequencing can be performed by detecting nucleotide addition. Nucleotideaddition can be detected using methods such as fluorescent emissionmethods or ion detection methods. For example, a set of fluorescentlylabeled nucleotides can be provided to the system 316 and can migrate tothe well 318. Excitation energy can be also provided to the well 318.When a nucleotide is captured by a polymerase and added to the end of anextending primer, a label of the nucleotide can fluoresce, indicatingwhich type of nucleotide is added.

In an alternative example, solutions including a single type ofnucleotide can be fed sequentially. In response to nucleotide addition,the pH within the local environment of the well 318 can change. Such achange in pH can be detected by ion sensitive field effect transistors(ISFET). As such, a change in pH can be used to generate a signalindicating the order of nucleotides complementary to the polynucleotideof the particle 310.

In particular, a sequencing system can include a well, or a plurality ofwells, disposed over a sensor pad of an ionic sensor, such as a fieldeffect transistor (FET). In embodiments, a system includes one or morepolymeric particles loaded into a well which is disposed over a sensorpad of an ionic sensor (e.g., FET), or one or more polymeric particlesloaded into a plurality of wells which are disposed over sensor pads ofionic sensors (e.g., FET). In embodiments, an FET can be a chemFET or anISFET. A “chemFET” or chemical field-effect transistor, includes a typeof field effect transistor that acts as a chemical sensor. The chemFEThas the structural analog of a MOSFET transistor, where the charge onthe gate electrode is applied by a chemical process. An “ISI-ET” orion-sensitive field-effect transistor, can be used for measuring ionconcentrations in solution; when the ion concentration (such as H+)changes, the current through the transistor changes accordingly.

In embodiments, the FET may be a FET array. As used herein, an “array”is a planar arrangement of elements such as sensors or wells. The arraymay be one or two dimensional. A one dimensional array can be an arrayhaving one column (or row) of elements in the first dimension and aplurality of columns (or rows) in the second dimension. The number ofcolumns (or rows) in the first and second dimensions may or may not bethe same. The FET or array can comprise 10², 10³, 10⁴, 10⁵, 10⁶, 10⁷ ormore FETs.

In embodiments, one or more microfluidic structures can be fabricatedabove the FET sensor array to provide for containment or confinement ofa biological or chemical reaction. For example, in one implementation,the microfluidic structure(s) can be configured as one or more wells (ormicrowells, or reaction chambers, or reaction wells, as the terms areused interchangeably herein) disposed above one or more sensors of thearray, such that the one or more sensors over which a given well isdisposed detect and measure analyte presence, level, or concentration inthe given well. In embodiments, there can be a 1:1 correspondence of FETsensors and reaction wells.

Returning to FIG. 3, in another example, a well 318 of the array ofwells can be operatively connected to measuring devices. For example,for fluorescent emission methods, a well 318 can be operatively coupledto a light detection device. In the case of ionic detection, the lowersurface of the well 318 may be disposed over a sensor pad of an ionicsensor, such as a field effect transistor.

One exemplary system involving sequencing via detection of ionicbyproducts of nucleotide incorporation is the Ion Torrent PGM™ orProton™ sequencer (Life Technologies), which is an ion-based sequencingsystem that sequences nucleic acid templates by detecting hydrogen ionsproduced as a byproduct of nucleotide incorporation. Typically, hydrogenions are released as byproducts of nucleotide incorporations occurringduring template-dependent nucleic acid synthesis by a polymerase. TheIon Torrent PGM™ or Proton™ sequencer detects the nucleotideincorporations by detecting the hydrogen ion byproducts of thenucleotide incorporations. The Ion Torrent PGM™ or Proton™ sequencer caninclude a plurality of template polynucleotides to be sequenced, eachtemplate disposed within a respective sequencing reaction well in anarray. The wells of the array can each be coupled to at least one ionsensor that can detect the release of H+ ions or changes in solution pHproduced as a byproduct of nucleotide incorporation. The ion sensorcomprises a field effect transistor (FET) coupled to an ion-sensitivedetection layer that can sense the presence of H+ ions or changes insolution pH. The ion sensor can provide output signals indicative ofnucleotide incorporation which can be represented as voltage changeswhose magnitude correlates with the H+ ion concentration in a respectivewell or reaction chamber. Different nucleotide types can be flowedserially into the reaction chamber, and can be incorporated by thepolymerase into an extending primer (or polymerization site) in an orderdetermined by the sequence of the template. Each nucleotideincorporation can be accompanied by the release of H+ ions in thereaction well, along with a concomitant change in the localized pH. Therelease of H+ ions can be registered by the FET of the sensor, whichproduces signals indicating the occurrence of the nucleotideincorporation. Nucleotides that are not incorporated during a particularnucleotide flow may not produce signals. The amplitude of the signalsfrom the FET can also be correlated with the number of nucleotides of aparticular type incorporated into the extending nucleic acid moleculethereby permitting homopolymer regions to be resolved. Thus, during arun of the sequencer multiple nucleotide flows into the reaction chamberalong with incorporation monitoring across a multiplicity of wells orreaction chambers can permit the instrument to resolve the sequence ofmany nucleic acid templates simultaneously.

FIG. 4 diagrammatically illustrates a system employing a valve, forexample, for carrying out pH-based nucleic acid sequencing. Eachelectronic sensor of the apparatus generates an output signal thatdepends on the value of a reference voltage. The fluid circuit permitsmultiple reagents to be delivered to the reaction chambers.

In FIG. 4, system 400 containing fluidics circuit 402 is connected byinlets to at least two reagent reservoirs (404, 406, 408, 410, or 412),to waste reservoir 420, and to biosensor 434 by fluid pathway 432 thatconnects fluidics node 430 to inlet 438 of biosensor 434 for fluidiccommunication. Reagents from reservoirs (404, 406, 408, 410, or 412) canbe driven to fluidic circuit 402 by a variety of methods includingpressure, pumps, such as syringe pumps, gravity feed, and the like, andare selected by control of valves 414. Reagents from the fluidicscircuit 402 can be driven through the valves 414 receiving signals fromcontrol system 418 to waste container 420. Reagents from the fluidicscircuit 402 can also be driven through the biosensor 434 to the wastecontainer 436. The control system 418 includes controllers for valves,which generate signals for opening and closing via electrical connection416.

The control system 418 also includes controllers for other components ofthe system, such as wash solution valve 424 connected thereto byelectrical connection 422, and reference electrode 428. Control system418 can also include control and data acquisition functions forbiosensor 434. In one mode of operation, fluidic circuit 402 delivers asequence of selected reagents 1, 2, 3, 4, or 5 to biosensor 434 underprogrammed control of control system 418, such that in between selectedreagent flows, fluidics circuit 402 is primed and washed, and biosensor434 is washed. Fluids entering biosensor 434 exit through outlet 440 andare deposited in waste container 436 via control of pinch valveregulator 444. The valve 444 is in fluidic communication with the sensorfluid output 440 of the biosensor 434.

FIG. 5, FIG. 6, FIG. 7, FIG. 8, FIG. 9, FIG. 10, and FIG. 11 illustratework pieces during a process for forming a microwell. For example, asillustrated in FIG. 5, a work piece 500 includes dielectric structure502 deposited over and surrounding a sensor pad 504 having a sensingsurface 506. The dielectric structure 502 can have more than one layer.For example, the illustrated dielectric structure 502 includes a highdensity plasma chemical vapor deposited silicon dioxide layer 508, asilicon nitride layer 510 deposited over the silicon oxide layer 508,and a TEOS silicon oxide layer 512 deposited over the silicon nitridelayer 510. Alternatively, the dielectric structure can be formed of asingle silicon oxide layer, a single silicon nitride layer, or onesilicon oxide layer and one silicon nitride layer. A photoresist layer514 can disposed over the dielectric structure 502 and can be patternedto defined photoresist openings 516 positioned over the sensor pads 504.

The work piece can be etched to form a first access at least partiallythrough the dielectric structure 502 above the sensor pads 504. Forexample, as illustrated in FIG. 6, a work piece 600 is etched to definea first access 618 at least partially through the dielectric structure502. In the illustrated example, the first access 618 extends through asilicon oxide layer 512. Alternatively, the first access 618 can extendinto the silicon nitride layer 510 or at least partially into a highdensity plasma chemical vapor deposited silicon dioxide layer 508. Asillustrated the first access 618 does not extend through the dielectricstructure to the sensor pad surface 506. Alternatively, the first access618 can be etched to contact the surface 506 of the sensor pad 504.

Etching can be implemented by a wet etch or a plasma etch. In anexample, etching includes plasma etching in chemistries selective to thelayer being etched.

The first access 618 can have a characteristic diameter. In an example,the characteristic diameter of the first access 618 can be in a range of0.05 μm to 0.5 μm, such as a range of 0.1 μm to 0.4 μm or a range of0.20 μm to 0.35 μm.

FIG. 7 illustrates the work piece 700 following aching and cleaning thephotoresist layer 514, for example, with an oxygen rich plasma orsolvent. As illustrated in FIG. 8, a work piece 800 includes aphotoresist layer 820 pattern with a photoresist opening 822. Theopening 822 is disposed over the first access 618 and over the sensorpad 504 and at least partially overlaps the first access 618 and thesensor pad 504 when observed from a top view. In a particular example,the opening 822 is concentric with an axis of the first access 618.

A second access can be etched into the work piece 800 resulting in awork piece 900 illustrated in FIG. 9. In an example in which the firstaccess 618 does not extend through to the dielectric structure 502 toexpose the sensor pad 504 following the first etching process, the firstaccess 618 can be extended to contact the surface 506 of the sensor pad504 forming an extended first access 926 between the second access 924and the sensor pad surface 506. In the illustrated example, the secondaccess 924 extends through the dielectric structure 502 but does notreach the sensor pad surface 506. For example, the second access 924 canextend through the low temperature silicon oxide layer 512, optionallythrough a silicon nitride layer 510, and may extend partially into ahigh density silicon oxide layer 508.

The second access 924 can have a characteristic diameter in a range of0.4 μm to 3.0 μm, such as a range of 0.4 μm to 2.1 μm, a range of 0.4 μmto 1.6 μm, a range of 0.5 μm to 1.1 μm, or a range of 0.6 μm to 0.8 μm.In particular, a diameter ratio between the characteristic diameter ofthe first access and the characteristic diameter of the second accesscan be in a range of 0.01 to 0.7. For example, the diameter ratiodefined as the ratio of the characteristic diameter of the first accessto the characteristic diameter of the second access can be in a range of0.05 to 0.6, such as a range of 0.1 to 0.6, or a range of 0.3 to 0.6.

FIG. 10 illustrates a work piece 1000 following aching and cleaning ofthe photoresist layer 820. For example, the work piece can be subjectedto an oxygen plasma treatment, followed by a vapor hydrogen fluoridetreatment and cleaning with a solvent, such as N-methyl pyrrolidone(NMP). As illustrated, the extended first access 926 and the secondaccess 924 form a microwell 1040.

Optionally, as illustrated in FIG. 11, a conformal metal layer 1132 canbe deposited into the microwell 1040 formed from the first access 926and the second access 924. For example, a metal, such as titanium,tungsten, gold, silver, tantalum, zirconium, aluminum, copper, hafnium,or a combination thereof, can be sputtered, deposited, or a combinationthereof to form a conformal coating 1132 over the surface of themicrowell 1040. The conformal metal coating 1132 can be removed frominterstitial areas 1130. For example, the wells 1040 can be filled withpolyimide followed by chemical mechanical polishing of both polyimideand metal from the interstitial spaces 1130. The polyimide canoptionally be removed by solvent, an oxygen plasma process, anadditional solvent bath, or a combination thereof.

In another example illustrated in FIG. 12, a work piece 1200 includes adielectric structure 1202 deposited over sensor pads 1204 having sensorsurfaces 1206. The dielectric structure 1202 can be formed of one ormore layers. In the illustrated example, the dielectric structure 1202includes a high density plasma chemical vapor deposited silicon dioxidelayer 1208, a silicon nitride layer 1210 deposited over the siliconoxide layer 1208, and a silicon oxide layer 1212 deposited over thesilicon nitride layer 1210. A photoresist layer 1214 can be formed andpatterned to define photoresist openings 1216. The dielectric structure1202 can then be etched to form a first access 1218 that extends throughthe dielectric structure 1202 to expose the sensor surface 1206. Thefirst access 1218 can have the characteristic diameter described abovein relation to the first access 618. Etching can include a wet etch or aplasma etch. In particular, etching includes a plasma etch.

As illustrated in FIG. 13, the photoresist layer 1214 can be cleanedfrom the surface, leaving the dielectric structure 1202 defining firstaccess 1218 exposing the sensor pad surface 1206.

As illustrated in FIG. 14, a bottom anti-reflective coating (BARC) 1420can be formed over the dielectric structure 1202, and a photoresistlayer 1422 can be formed over the bottom anti-reflective coating (BARC)layer 1420. The photoresist layer 1422 and optionally the BARC layer1420 can be patterned to define photoresist openings 1424. The BARClayer 1420 at least partially deposits into the first access 1218. Thephotoresist openings 1424 can overlap the first access 1218 and thesensor pad 1204 from a top view. In particular, the photoresist opening1424 can be concentric with an axis of the first access 1218.

As illustrated in FIG. 15, a second access 1526 can be etched into thedielectric structure 1202. For example, the second access 1526 can beetched using a plasma etch. At least a portion of the BARC layer 1420remains in the first access 1218, potentially protecting the sensor padsurface 1206 from exposure to the plasma etching of the second access1526.

The second access 1526 can have a characteristic diameter similar to thecharacteristic diameter described above in relation to the second access924 described above. Further, the diameter ratio of the characteristicdiameter of the first access 1218 to the characteristic diameter of thesecond access 1526 can have can have the diameter ratio described abovein relation to the first access 618 and the second access 924 describedabove.

As illustrated in FIG. 16, the photoresist layer 1422 and the BARC layer1420 can be cleaned from the surface. For example, the layers can becleaned using an oxygen plasma, a vapor hydrogen fluoride treatment, anNMP solvent, or a combination thereof. The first access 1218 and thesecond access 1526 form a well 1640.

In a further example illustrated in FIG. 17, a conformal metal coating1732 can be deposited into the wells 1640 formed by the first access1218 and the second access 1526. For example, a metal layer can bedeposited using sputtering or a vapor deposition or a combinationthereof followed by providing a protective polymer coating andperforming chemical mechanical polishing to remove the metal layer frominterstitial areas 1730.

The polymer coating can be removed using solvent treatments, plasmaashing methods or a combination thereof. In an alternative exampleillustrated in FIG. 18, the polymer coating can remain in place in thewells while an additional dielectric structure 1834 is deposited overthe dielectric structure 1202. For example, an additional oxide layer1834 can be deposited over the dielectric layer 1202 using plasmaenhanced chemical vapor deposition (PECVD). Photoresist can be formedover the oxide layer 1834 and the photoresist layer can be patterned todefine further openings overlapping with the first access and the secondaccess. An additional access can be etched through the oxide layer 1832to provide a third access 1836 in fluid communication with the secondaccesses 1526. The protective polymer in the second access 1526 and thefirst access 1218 can be removed along with the photoresist layer usingsolvent sprays, oxygen plasma ashing, an NMP bath, or a combinationthereof. While the formation of the additional oxide layer 1832 isdescribed in relation to the process illustrated in FIGS. 12 through 17,such a layer can also be formed following the process illustrated inFIGS. 5 through 11.

The characteristic diameter of the third access 1836 can be in a rangeof 0.4 μm to 3 μm, such as a range of 0.4 μm to 2.1 μm, a range of 0.4μm to 1.6 μm, a range of 0.5 μm to 1.1 μm, or a range of 0.6 μm to 0.8μm. The characteristic diameter of the third access 1836 can be similarto the characteristic diameter of the second access 1526. In anotherexample, the characteristic diameter of the third access 1836 can besmaller than the characteristic diameter of the second access 1526.

The above described methods yield desirable technical advantages. Theetch chemistry can be selected to etch silicon oxides and nitrides whileminimally affecting photoresist. Further, etching the second accessproceeds to the desired depth of the well and not to the depth of thesensor surface. The photoresist can better tolerate such an etchprocess. In addition, when using a BARC layer, the sensor pad is notexposed during the second etch process.

Note that not all of the activities described above in the generaldescription or the examples are required, that a portion of a specificactivity may not be required, and that one or more further activitiesmay be performed in addition to those described. Still further, theorder in which activities are listed are not necessarily the order inwhich they are performed.

In the foregoing specification, the concepts have been described withreference to specific embodiments. However, one of ordinary skill in theart appreciates that various modifications and changes can be madewithout departing from the scope of the invention as set forth in theclaims below. Accordingly, the specification and figures are to beregarded in an illustrative rather than a restrictive sense, and allsuch modifications are intended to be included within the scope ofinvention.

As used herein, the terms “comprises,” “comprising,” “includes,”“including,” “has,” “having” or any other variation thereof, areintended to cover a non-exclusive inclusion. For example, a process,method, article, or apparatus that comprises a list of features is notnecessarily limited only to those features but may include otherfeatures not expressly listed or inherent to such process, method,article, or apparatus. Further, unless expressly stated to the contrary,“or” refers to an inclusive-or and not to an exclusive-or. For example,a condition A or B is satisfied by any one of the following: A is true(or present) and B is false (or not present), A is false (or notpresent) and B is true (or present), and both A and B are true (orpresent).

Also, the use of “a” or “an” are employed to describe elements andcomponents described herein. This is done merely for convenience and togive a general sense of the scope of the invention. This descriptionshould be read to include one or at least one and the singular alsoincludes the plural unless it is obvious that it is meant otherwise.

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any feature(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature of any or all the claims.

After reading the specification, skilled artisans will appreciate thatcertain features are, for clarity, described herein in the context ofseparate embodiments, may also be provided in combination in a singleembodiment. Conversely, various features that are, for brevity,described in the context of a single embodiment, may also be providedseparately or in any subcombination. Further, references to valuesstated in ranges include each and every value within that range.

What is claimed is:
 1. A method for forming a well providing access to asensor pad, the method comprising: patterning a first photoresist layerover a dielectric structure disposed over the sensor pad; etching afirst access into the dielectric structure and over the sensor pad, thefirst access having a first characteristic diameter; patterning a secondphotoresist layer over the dielectric structure; and etching a secondaccess over the dielectric structure and over the sensor pad, the secondaccess having a second characteristic diameter, the first and secondaccesses overlapping, a diameter ratio of the first characteristicdiameter to the second characteristic diameter being not greater than0.7, the first access exposing the sensor pad, the second access havinga bottom depth less than a bottom depth of the first access.
 2. Themethod of claim 1, wherein the diameter ratio is in a range of 0.01 to0.7.
 3. The method of claim 2, wherein the diameter ratio is in a rangeof 0.05 to 0.6.
 4. The method of claim 3, wherein the diameter ratio isin a range of 0.1 to 0.6.
 5. The method of claim 4, wherein the diameterratio is in a range of 0.3 to 0.6.
 6. The method of claim 1, whereinetching the first access includes etching the first access to a bottomdepth to expose the sensor pad.
 7. The method of claim 1, whereinetching the first access includes etching the first access to a bottomdepth that does not expose the sensor pad.
 8. The method of claim 1,wherein the dielectric structure includes an oxide layer and a nitridelayer.
 9. The method of claim 8, wherein the nitride layer is disposedover the oxide layer.
 10. The method of claim 8, wherein the oxide layeris a high density plasma chemical vapor deposited silicon dioxide layer.11. The method of claim 8, wherein the dielectric structure furtherincludes a tetraethyl orthosilicate deposited oxide layer.
 12. Themethod of claim 11, wherein the low temperature oxide layer is disposedover the nitride layer.
 13. The method of claim 11, wherein etching thefirst access includes etching the first access through the lowtemperature oxide layer, the nitride layer and the oxide layer.
 14. Themethod of claim 11, wherein etching the first access includes etchingthe first access through the low temperature oxide layer and not theoxide layer.
 15. The method of claim 11, wherein etching the secondaccess includes etching the second access through the low temperatureoxide layer and the nitride layer.
 16. The method of claim 15, whereinetching the second access includes etching the second access partiallyinto the oxide layer.
 17. The method of claim 1, further comprisingcleaning with an oxygen ash step following etching the second access.18. The method of claim 17, further comprising performing a vaporhydrogen fluoride treatment following the oxygen ash step.
 19. Themethod of claim 1, further comprising depositing a conformal metalcoating over the first and second accesses following etching the secondaccess.
 20. The method of claim 19, further comprising removing theconformal metal coating from interstitial areas over the dielectricstructure.